1. Area of the Invention
This invention relates to spread spectrum communications systems and more particularly to optical spread spectrum communications systems.
2. Description of the Prior Art
Over the last two decades, optical communications over fiber optics have come increasingly into use for transmission of large quantities of data. Although initially, such fiber systems used narrow bandwidth systems such as those using laser light for communications, attempts have been made to increase system capacities. For example, in prior art systems, wavelength (frequency) division multiplexing has been accomplished by modulating a plurality of lasers at different frequencies and transmitting the modulated light from the various different lasers over the same fiber. Still further, various forms of time domain multiplexing have been done over digital fiber networks.
There have been a few proposals to provide digital optical networks using spread spectrum communications. These are set forth in the following papers and presentations: "Coherent Ultrashort Light Pulse Code-Division Multiple Access Communication Systems", Journal of Lightwave Technology, Vol. 8, No. 3, March 1990; L. Nguyen, B. Aazhang, J. F. Young "Optical CDMA with Spectral Encoding and Bipolar Codes", Proc. 29th Annual Conf. Information Sciences and Systems (Johns Hopkins University, Mar. 22-24, 1995); N. B. Mandayan, B. Aazhang, "An Adaptive Single-user Detector for Optical Code Division Multiple Access Systems," Proc. 28th Annual Conf. Information Sciences and Systems, (Princeton University N.J. Mar. 16-18 1994) M. Brandt-Pearce, B. Aazhang, "Performance of Multiuser Detection for Optical Spectral Amplitude CDMA System", Proc. 27th Annual Conf. Information Sciences and Systems (Johns Hopkins University Mar. 24-26 1993) p. 308-11; N. B. Mandayam, B. Aazhang "Generalized Sensitivity Analysis for Optical Code Division Multiple Access Systems" Proc. (same) p. 302-07; M. Brandt-Pearce, B. Aazhang "Optical Spectral Amplitude Code Division Multiple Access System" Proc. International Symposium on Information Theory, San Antonio Tex. p. 379 Jan. 17-22, 1993; M. Brandt-Pearce et al. "Performance Analysis of Single-user and Multiuser Detectors for Optical Code Division Multiple Access Communications," IEEE Transactions on Communications, Vol. Com-43 No. 3 1995; A. Pasasakellariou et al. "Code Design for Interference Suppression in CDMA Systems with Continuous Phase Modulation" Proc. 29th Annual Conf. Information Sciences and Systems, Johns Hopkins U. Md. 1995; and "High Capacity Optical CDMA Communications Networks presented July 1994 at the assignee of this application; A Semiclassical Analysis of Optical Code Division Multiple Access, D. Brady and S. Verdu, IEEE Transactions on Communications, Vol. 39, No. 1, January 1991, pp. 85-93; W. C. Wong et al. "Synchronous vs. Asynchronous CDMA for Fiber Optic LANS Using Optical Signal Processing", November 1989, pp. 1012-1016; and the following U.S. Pat. No. 5,519,526 to Chan et al.; U.S. Pat. No. 4,703,474 to Foschini et al.; U.S. Pat. No. 5,289,299 to Paek et al.; U.S. Pat. No. 5,499,236 to Giallorenzi et al.; U.S. Pat. No. 5,410,147 to Riza et al.; and U.S. Pat. No. 5,438,440 to Paek et al.
In such prior art CDMA designs, however, there are severe drawbacks in the number of simultaneous users that the system can support. In particular, to efficiently use CDMA in optical communications, a large number of orthogonal codes must be used for providing the various coded channels. Preferably, it is desirable to have several hundreds or more such coded channels in a network. However, in optical communication a drawback of using a large number of codes is that this raises the effective noise floor in the system. In CDMA, each uniquely coded channel constitutes noise for the other channels and as the number of channels increases, this noise level increases dramatically.
Various systems have been tried to increase the number of channels. For example U.S. Pat. No. 5,499,236 proposes using synchronous transmission to reduce the noise and using pseudo noise codes that modulate the data to spread the data spectrum. However, such pseudo noise codes are not orthogonal and the spreading is in the electrical domain. Further, the requirement of synchronization requires a master station with a precise reference clock and signaling of time information back to the transmitters. U.S. Pat. No. 4,703,474 proposes combining spread spectrum techniques in the electrical domain with wavelength (frequency) division multiplexing in the optical domain. Here, pseudo-noise (PN) codes are used in the time domain and thus the system suffers the same problem as U.S. Pat. No. 5,499,236. In addition, such a system requires complex time and frequency sweeps for acquisition of signal. U.S. Pat. No. 5,438,440 suggests the use of a monochromatic light signal and a two-dimensional spatial digital encoder; i.e., a bit in the mask pattern is either fully transparent or fully opaque. Although such two dimensional spatial encoding improves the number of users the network can support relative to other prior art, it requires very complicated holographic detection systems. Using holographic detection, it is difficult to change the receive hologram so that if stations in a optical spread spectrum network are removed from the network for maintenance or other reasons, the receive code cannot be readily reassigned to a different node, lowering overall efficiency.
One proposal has been to use bipolar digital codes as described in "Optical CDMA with Spectral Encoding and Bipolar Codes" cited above. In these bipolar digital codes, bipolar Walsh or Gold codes of length N are generated for a transmitter code. For a given code U, the complement U* is also generated and the code and the complement are concatenated together as U.sym.U* to form an encoding code for first state. This code is embedded in the mask along an axis with for example one state in the digital code for a cell being implemented as a transparent area and zeros represented by an opaque area in the mask. In this system, a broad band light source is dispersed with a dispersion grating, collimated with a collimating lens, passed through the mask for spatial encoding so that the axis that the light is spread is arranged along the axis of the code, focused back on a recombining grating, and then provided to the modulator. A second, almost identical encoder is also provided for encoding light from the same light source with the complementary code U*.sym.U, which encoded light is also provided to the modulator. Based upon input data or some other information source, the modulator selects between the encoded spectrum from the two different sources to provide an encoded, modulated spread spectrum with U.sym.U* representing a data bit "one" and U*.sym.U representing a data bit "zero." One mask can be used to generate both codes (i.e., U.sym.U* and U*.sym.U) by stacking the two codes on the same mask pattern. Alternatively a reflecting mask may be used to generate the two codes for the modulator.
Decoding according to this technique requires transmission of the two codes (U.sym.U* and U*.sym.U) on different channels and then each of the codes is passed through separate matched filter for both U.sym.U* and U*.sym.U. The output of the matched filters for each of the codes is then supplied to optical adders and then to photodetectors in a differential arrangement.
It is believed, however, that this technique using bipolar, concatenated codes and their complements will not permit less than an optimal number of users in a real life system as the interference from each of the users is believed to be high. Further, the transmitting of the two codes in separate, recoverable channels imposes costs on the system and can also result in different path delays and transmit channel mismatch.
Therefore, it is a first object of the invention of having a spread spectrum communication system where the number of users is maximized without raising interference unduly. It is a second object of the invention to provide a system providing a relatively simple system for encoding and decoding the light but efficiently using the entire spectrum available. It is yet a third object of the invention to provide a spread spectrum communication system having codes that may be readily reassigned to other transmitters in the network.